This application relates generally to combustors and, more particularly, to an acoustic damper in a combustion liner of the combustor of a gas turbine engine.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Pollutant emissions from industrial aero engines are subject to Environmental Protection Agency (EPA) standards that regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). In general, engine emissions fall into two classes: those formed because of high flame temperatures (NOx), and those formed because of low flame temperatures that do not allow the fuel-air reaction to proceed to completion (HC & CO).
More stringent emission regulations have led to gas turbine combustion systems that utilize fuel-lean premixed combustion. Lean flames significantly reduces NOx emissions due to lower flame temperature, but are more sensitive to combustion acoustics, which may limit operability and performance, impact pollutant emissions, and reduce the useful life of combustor components. Typically, great effort is taken to optimize combustor design to meet both emissions and operability requirements.
At least some known gas turbine combustors include a plurality of mixers which mix high velocity air with liquid fuels, such as diesel fuel, or gaseous fuels, such as natural gas, to enhance flame stabilization and mixing. At least some known mixers include a single fuel injector located at a center of a swirler for swirling the incoming air. Both the fuel injector and mixer are located on a combustor dome. The combustor includes a mixer assembly and a heat shield that facilitates protecting the dome assembly. The heat shields and combustor liner are cooled by air impinging on the dome to facilitate maintaining operating temperature of the heat shields within predetermined limits.
During operation, the expansion of the mixture flow discharged from a pilot mixer may generate toroidal vortices around the heat shield. Unburned fuel may be convected into these unsteady vortices. After mixing with combustion gases, the fuel-air mixture ignites, and an ensuing heat release can be very sudden. In many known combustors, hot gases surrounding heat shields facilitate stabilizing flames created from the ignition. However, the pressure impulse created by the rapid heat release can influence the formation of subsequent vortices, and cause repetitive stress fatigue to combustor components. Subsequent vortices can lead to pressure oscillations within combustor that exceed acceptable limits, and cause repetitive stress fatigue to combustor components.
Combustion acoustics in gas turbine engines can occur over a range of frequencies. Typical frequencies are less than 1000 Hz. However under certain conditions high acoustic amplitudes for frequencies in the 1000˜5000 Hz range are possible. These high-frequency acoustic modes can cause rapid failure of combustor hardware due to high cycle fatigue. The increase in energy release density and rapid mixing of reactants to minimize NOx emissions in advanced gas turbine combustors enhance the possibility of high frequency acoustics.